(Description of Terminology)
First, terminology used in the present invention is described.
<Recording Head>
A recording head records an image onto a recording body with a recording beam. For example, when the energy source is light, a laser beam irradiated from a laser light source is focused on the recording body with a lens. An image is formed by turning the laser beam on/off, forming portions on the recording body that react to light and portions that do not receive light.
An example of a recording head is shown in FIG. 1. The recording head shown in FIG. 1 includes a semiconductor laser LD 1, an aspherical lens 2, a diaphragm 3, and an adhesive 4.
<Recording Beam>
The recording head shown in FIG. 1 uses a laser beam as a recording beam. Generally, a recording beam records an image by transferring light, heat, impacts of a substance, or a substance itself such as ink, to a recording body.
<Recording Body>
A recording body reacts to energy from the recording head, and indicates different physical features at portions where energy is irradiated and portions where energy is not irradiated, thereby recording an image. For example, an image is recorded by chemical reaction, changes in phases, or changes in shape. Specifically, a recording body that uses light energy is made of a photosensitive material for reacting to light energy, a heat-sensitive material for reacting to heat of a laser beam, or reaction material that burns due to heat of a laser beam.
<Original Image Data>
Original image data represent an image to be formed by an image forming apparatus. For example, an image may be expressed by a page description language that specifies figures with characters formed by parameters of equations for dots and surfaces and parameters specifying character string codes and font types. Other examples are bitmap data of an arbitrary resolution or data of a page description language including bitmap data.
<Rasterization>
Rasterization means converting original image data to a set of dots (set of bits) that an image forming apparatus can record on a recording body. As a result of the conversion, 1 bit of rasterized data is recorded on the recording body as 1 dot. To output halftones, grayscales are converted to halftone dots, corresponding to a predetermined number of dots per unit area.
<Positional Displacement Information>
As shown in FIG. 2, when recording positions of recording heads are at ideal positions, recording regions of each of the recording heads on a recording body are arranged continuously with adjacent regions. However, in reality, due to manufacturing variations, the recording positions of the recording heads are arranged inconsecutively on the recording body, as indicated by solid lines shown in FIG. 3. Thus, an amount of positional displacement (x, y) between an ideal recording region and an actual recording region is obtained, as shown in FIG. 4. In this example, the ideal recording region is rectangular; a positional displacement amount can be a distance that vertex coordinates have moved. In this example, positional displacements from ideal vertex positions are obtained. However, in effect, as long as adjacent recording regions are arranged continuously and are not displaced from each other, the recorded image appears fine. Accordingly, the positional displacement amount can be a relative distance between pixels of adjacent recording regions, which pixels are ideally adjacent to each other. Regardless of how the displacement amount is expressed, positional displacement information represents an inconsecutive region, i.e., a gap appearing at a boundary between adjacent regions corresponding to adjacent recording heads in a recorded image.
The positional displacement information varies between different image forming apparatuses. Accordingly, a reference image (marker) is plotted on the recording body, and positional displacement information is obtained based on the plotted reference image.
In FIG. 5, marks of an original (M1, M2) are recorded on a recording body. Each mark is recorded in one of the recording regions of two recording heads. The marks recorded by the two recording heads are compared with original marks (marks on the original), so as to detect a positional displacement therebetween.
In FIG. 5, the positional displacement is detected from a positional relationship v0 between marks on the original and a positional relationship v1 between marks recorded on the recording body. Thus, a relative positional displacement between the two recording heads can be detected.
<Scanning Information>
Scanning information corresponds to data expressing a position at which image data are to be recorded when forming an image. When there are N scanning lines, scanning positions from the left are expressed as L [1], L [2] . . . L[N]. Normally, position information is expressed as 1, 2, 3 . . . N for L[1], L[2] . . . L[N].
In order to increase scanning density, three scanning lines evenly spaced apart can be added in between scanning line L[1] and scanning line L[2], for example. When the added scanning lines are included, the positional information for L[1], L[2], L[3] . . . is 1, 1.25, 1.5, 1.75, 2, 3 . . . N.
The scanning information also includes height information Lh for determining a position from which scanning starts (scanning start position), to be described below.
<Step Scanning>
As shown in FIG. 6, in a step scanning method, a movable stage 15 stops while a recording body 11 wrapped around a rotating drum 12 is facing recording heads 16. The movable stage 15 moves to the next scanning position when a non-recording portion of the rotating drum 12 is facing the recording heads 16.
<Spiral Scanning>
As shown in FIG. 7, in a spiral scanning method, the movable stage is constantly moving while the drum is rotating. Normally, the movable stage moves at a speed such that one main scanning line is scanned during one rotation of the drum. Accordingly, the surface of the drum can be scanned in a spiral manner.
(Conventional Image Forming Apparatus)
Next, an example of a conventional image forming apparatus is described with reference to FIG. 6.
The image forming apparatus employing the step scanning method shown in FIG. 6 includes the recording body 11, the drum 12, a drum encoder 14, the movable stage 15 that moves in parallel with the drum 12, the recording heads 16 provided on the movable stage 15, and a rotational axle 17. Recording beams irradiated from the recording heads 16 scan the recording body 11 to form an image.
The recording body 11 is a recording material used for image formation, and is wrapped around the surface or the underside of the circumference of the drum 12. The recording body 11 is fixed to the circumferential surface of the drum 12 with a fixing mechanism such as a clamping mechanism. The drum 12 is rotatable around the rotational axle 17, and is rotated by not shown driving means attached to the rotational axle 17. In order to accurately control the rotation of the drum 12, a stepping motor or a servo motor is employed as the driving means.
The drum encoder 14 is provided on one end of the drum 12. The drum encoder 14 includes a light source and a light detecting device that detects light irradiated from the light source, so as to detect the rotational position of the rotating drum 12. Further, the drum encoder 14 can detect the home position of the drum 12, i.e., the position from which the drum 12 starts rotating.
The movable stage 15 is movable in the axial direction of the drum 12, under control of a ball screw or a linear motor. A scan trajectory 13 moves in accordance with the movement of the movable stage 15.
The image forming apparatus shown in FIG. 6 operates as follows.
The drum 12 is rotated by a power source such as a motor. As described above, the drum encoder 14 detects the rotational position of the drum 12. Specifically, positions of the recording body 11 and the recording heads 16 can be obtained from output from the drum encoder 14. Based on the obtained positions, a recording timing to perform recording onto the recording body 11 is determined.
The image forming apparatus detects the home position of the drum 12 with the drum encoder 14, and the recording heads 16 start recording an image. With one rotation of the drum 12, each recording head 16 scans one line. This is referred to as main scanning.
When one main scanning operation on the recording body 11 is completed, the movable stage 15 moves horizontally to the position of the next main scanning operation; this is referred to as sub scanning. Subsequently, main scanning is performed. Recording beams from the recording heads 16 scan the recording body 11 by alternately repeating sub scanning and main scanning. When scanning of a predetermined region on the recording body 11 is completed, the process of creating an image is completed.
In the above example, sub scanning is performed every time the drum 12 rotates once, i.e., in a stepwise manner. Instead of a stepwise manner, it is also possible to perform sub scanning substantially continuously, so that the recording body 11 is scanned in a spiral manner. The image forming apparatus described with reference to FIG. 7 performs sub scanning in a spiral manner. In the image forming apparatus described with reference to FIG. 7, the movable stage that moves the recording heads is constantly moving at a speed such that one main scanning line is scanned during one rotation of the drum.
(Conventional Technology)
A technology disclosed in Japanese Laid-Open Patent Application No. 2001-88346 (Patent Document 1) is described with reference to FIG. 8. A laser beam L1 and a laser beam L2 irradiated from adjacent recording heads continuously record images in recording regions A1 and A2. In a recording region C12, the number of main scanning lines recorded by the laser beam L1 is gradually reduced, while the number of main scanning lines recorded by the laser beam L2 is gradually increased, so that the boundary between adjacent recording regions A1, A2 in the image is inconspicuous.
In an invention described in Japanese Laid-Open Patent Application No. 2002-72494 (Patent Document 2), an image is divided into plural segments to be recorded by plural laser beams, and the sub scanning speed is reduced near boundaries of adjacent images so as to adjust intervals between main scanning lines. The main scanning lines are divided in the main scanning direction, and are separated and formed in a sub scanning direction, so that differences between inclinations of the main scanning lines are eliminated. Accordingly, high quality images can be recorded at high speed.
In an invention described in Japanese Laid-Open Patent Application No. 2004-147260 (Patent Document 3), when one set of original image data is divided so that image formation is performed by plural recording heads, positional displacements of the divided parts can be corrected by a simple method. Specifically, a single set of image data can be divided into plural parts based on image regions corresponding to the recording heads, so as to create divided image data. According to positional displacements of the divided images, a new correction image data area is additionally provided based on the divided image data and detection results of positional displacement amounts. The divided image data are arranged in the correction image data area based on positions obtained from detection results of the positional displacement amounts. Thus, positional displacements between divided images are prevented.
In an invention described in Japanese Patent No. 3604961 (Patent Document 4), a print region on a recording medium or an intermediate recording medium, in which image information is actually recorded, is divided into at least two segments. The segments are superposed onto each other at boundary parts. A relative positional difference detecting unit exposes three or four positional marks onto an exposure area including the superposed regions, and calculates a positional displacement amount of the exposure area from a detected value of a positional displacement amount between the positional marks. Image information forming units form image information based on positional displacement amounts of the exposure area. An image information correcting unit corrects the image information so as to match the actual print region.
Patent Document 1: Japanese Laid-Open Patent Application No. 2001-88346
Patent Document 2: Japanese Laid-Open Patent Application No. 2002-72494
Patent Document 3: Japanese Laid-Open Patent Application No. 2004-147260
Patent Document 4: Japanese Patent No. 3604961
In the invention described in Japanese Laid-Open Patent Application No. 2001-88346, in a recording region where images recorded by, adjacent recording beams are superposed, the number of main, scanning lines recorded by one laser beam is gradually reduced, while the number of main scanning lines recorded by another laser beam is gradually increased, so that the boundary between adjacent images is inconspicuous. However, in this method, intervals between scanning lines from the two laser beams are not adjusted at all. Therefore, if a positional displacement between the two laser beams is half of the scanning intervals, stripes may appear at boundaries between scanning lines from different laser beams. In such a case, as there are boundaries throughout the entire superposed region, the number of stripes is increased, and image quality is degraded.
In the invention described in Japanese Laid-Open Patent Application No. 2002-72494, the sub scanning speed is reduced near boundaries of adjacent images to adjust intervals between main scanning lines, so that inconsecutive portions at boundaries are inconspicuous. However, in order to reduce the sub scanning speed in spiral scanning, extra processes are necessary to eliminate differences between inclinations of the main scanning lines. Specifically, the processes include dividing the main scanning lines in a main scanning direction so as to be separated and formed in a sub scanning direction. Further, when performing processes to correct inclinations of plural main scanning lines, interference may occur between the number of main scanning lines subject to inclination correction and periods of area modulation patterns, used for expressing image density. Accordingly, stripes may be visible at boundaries of images. Further, by reducing the sub scanning speed, the friction resistance of stage machine parts for sub scanning, e.g., a guide rail, deviates from normal values. Accordingly, the driving torque of the driving source deviates from normal values. Thus, precision of scanning positions varies between segments scanned at normal speed and segments scanned at reduced speed; therefore, fine stripes may be visible in the resultant image.
In the invention described in Japanese Laid-Open Patent Application No. 2004-147260, embedded images are provided for each recording head to measure positional displacements, which makes the structure complex. Further, fractional parts of positional displacements are not taken into account; therefore, the positional displacements are not thoroughly corrected.
The invention described in Japanese Patent No. 3604961 involves exposing three or four positional marks onto the exposure area, which makes the structure complex.
Accordingly, there is a need for an image forming apparatus, an image forming method, and an image forming program product in which positional displacements of images recorded by adjacent recording heads can be corrected in main scanning and sub scanning directions without changing the sub scanning speed, and differences in recording densities between recording heads are not visible in recorded images.